Recovery-annealed cold-worked titanium steels



Jan. 27,1970 H. GOODENOW 3,492,173

RECOVERY-ANNEALED COLD-WORKED TITANIUM STEELS Filed July 21, 196 2 Sheets-Sheet 1 850F 0.9 w e g Q IO50F TENSILE STRENGTH ANNEALED TENSILE STRENGTH COLD WORKED I I I I- I OWOJ I10 I0 :00 I000 TIME AT TEMPERATURE, MINUTES Fig! % ELONGATION TENSILE STRENGTH ANNEALED TENSILED STRENGTH COLD WORKED 5 "/0 ELONGATION O 0.! L0 I0 100 1000 TIME AT TEMPERATURE, MINUTES INVENTOR.

.2 ROBERT H. GOODENOW g BY ATTORNEY Jan. 27, .1910

Filed July 21, 1967 R. H. GOODENOW 3,492,173

RECOVERY-ANNEALE'D COLD-WORKED T I TAN I UM STEELS 2 Sheets-Sheet 2 IO TIME AT TEMPERATURE, MINUTES l I l ITO 2 Q w v N O N) lSd Ol'HiSNBHlS 3'HSN31 (we) NouvsNo-lg 19d INVENTOR' d, 2 ROBERT H. coooswow L| BY jfii w;

ATTORNEY United States Patent 3,492,173 RECOVERY-ANNEALED COLD-WORKED TITANIUM STEELS Robert H. Goodenow, Bethel Park, Pa., assignor to Jones & Laughlin Steel Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed July 21, 1967, Ser. No. 655,089 Int. Cl. C21d 7/14, 7/02 US. Cl. 148-12 6 Claims ABSTRACT OF THE DISCLOSURE According to the present invention, plain low-carbon titanium-bearing steels having effective titanium to carbon ratios of at least 4 to 1 are cold worked and thereafter recovery annealed to produce steel products having desirable physical properties. These steels show an increase in ductility during such a recovery anneal regardless of the time and temperature of the anneal.

This invention relates to a method for processing titanium-bearing steels having effective titanium to carbon ratios of at least 4 to 1, so as to produce steel products having generally higher tensile strengths and yield strengths, and higher ductilities at given tensile strengths than similar steels containing no titanium processed in a like manner.

Steels containing at least four times as much effective titanium as carbon are known to possess certain distinctive properties; for example, they exhibit no strain aging or discontinuous yielding and demonstrate generally good drawing characteristics. I have now discovered that these steels, when subjected to a higher degree of cold reduction and thereafter recovery annealed, provide steel products of generally higher tensile strengths, yield strengths and higher ductilities than similarly processed plain low-carbon killed or rimmed steels containing no titanium, and, in addition, show an increase in ductility during such an anneal independent of the time and temperature of heating and independent of the extent of cold reduction. These physical properties make the steel products especially suited for application where good formability, high tensile strength and high yield strength is required, such as in beverage can end plate. I have further discovered that any particular titanium steel manifests the same percent elongation at a given tensile strength for the same degree of cold working after a recovery anneal regardless of the annealing temperature. Thus the same degree of ductility and corresponding tensile strength may be imparted to any given titanium steel after a cold reduction by either a batch or continuous recovery anneal.

As indicated the present invention is limited to the processing of titanium-bearing steels wherein the effective titanium to carbon ratios are greater than 4 to 1. This ratio is. based on the amount of titanium in the steel in excess of the amount necessary to combine with all the sulphur, nitrogen and oxygen therein and is present either combined with carbon or in an uncombined state. Because titanium unites preferentially with oxygen, nitrogen and sulphur with respect to carbon, the steels processed "ice according to the present invention are generally aluminum killed or degassed so that the amount of titanium required to provide an effective titanium to carbon ratio of greater than 4 to l is not excessive.

Briefly, the present invention comprises the steps of cold reducing a titanium-bearing steel having the requisite amount of titanium to final gauge and thereafter subjecting the cold worked product to a recovery anneal. As used herein, the term recovery annea means a heat treatment wherein essentially no recrystallization of the steel occurs.

The invention will now be described in greater detail with reference to the accompanying drawings in which:

FIGURE 1 is a graphical representation of the change in tensile strength and ductility, as measured by percent elongation, of a 50% cold reduced titanium-bearing steel during annealing.

FIGURE 2 is a graphical representation of the same relationship illustrated in FIGURE 1 wherein the steel has been cold reduced FIGURE 3 is a graphical representation of the change in tensile strength and percent elongation during annealing at 850 F. of a titanium bearing steel and a similar plain low-carbon rimmed steel containing no titanium wherein the steels have been subjected to varying degrees of cold reduction.

A plain low-carbon aluminum killed steel containing titanium was made into an ingot according to commercial steelmaking practices. The steel had the following composition: 0.040% C, 0.44% Ti, 0.34% Mn, 0.025% A1, 0.003% P, 0.011% S and 0.030% Si. A total of about 068% of the titanium was found to be present in the steel as titanium oxide, titanium nitride and titanium sulphide. Thus about 0.372% of the titanium was available to combine with the carbon so that the effective titanium to carbon ratio of the steel was 9.3 to 1. The steel was hot rolled with a finishing temperature between 1700 F. and 1750 F. and one portion of the hot band, referred to herein as T 1, was then cold rolled 50% and another portion, referred to herein as T2, was cold rolled 85%. Upon the cold rolling, steel T1 had a longitudinal tensile strength of 95,400 p.s.i. and a percent elongation (2") of about 3.0% while steel T2 had a longitudinal tensile strength of 116,700 p.s.i. and a percent elongation (2") also of about 2.5%. Samples of both steel T1 and steel T2 were then annealed isothermally at various temperatures between 850 F. and 1350 F. The changes in tensile strength and percent elongation with time during the course of the various anneals are shown in FIGURE 1 for T1 and in FIG- URE 2 for T2.

The upper portion of FIGURE 1 contains a plot of the change in the recovery fraction during the annealing of T1 at various temperatures. The recovery fraction is the quotient of the tensile strength of T1 at any particular point in the heat treatment divided by the tensile strength of T1 in the cold rolled condition. As can be seen, after an initial rapid recovery from a recovery fraction value of 1, the tensile strength of T1 decreases at a uniform exponential rate with time at all annealing temperatures until a recovery fraction of about 0.8 is reached, i.e., until there is about a 20% recovery in strength. Thereafter the tensile strength recovers at a much faster rate as a function of time. The start of this more rapid recovery corresponds to the start of recrystallization of the steel. During the period of the uniform exponential decrease is strength, a corresponding uniform exponential increase in percent elongation occurs. This is illustrated in the bottom portion of FIGURE 1 where percent elongation as an exponential function of time is plotted for the heat treatments carried out at 1150 F. and 850 F.

Titanium-bearing steels having a titanium to carbon ratio greater than 4 to 1 are unique in that upon being processed as described, the percent elongation exhibited by such steels is approximately the same for any given tensile strength regardless of the annealing temperature. Thus with reference to FIGURE 1, it can be seen that at a recovery fraction of 0.89, the percent elongation exhibited by the steel is about 8% at both an annealing temperature of 850F. and 1150 F. The time required to reach that point is, however, different at the different temperatures. At 850 F. it takes approximately 1,200 minutes and at 1150 F. it requires .2 minutes. As a result, a particular tensile strength and percent elongation can be obtained for the titanium steels either by a continuous recovery anneal or by a batch recovery anneal. Thus a greater degree of flexibility in operating conditions is available when using these steels. This property is absent in plain low-carbon rimmed or killed steels having no titanium or a titanium-carbon ratio less than about 4 to 1.

FIGURE 2 illustrates that steel T2 displayed the same general relationship between time, temperature, tensile strength and percent elongation as steel T1. However, because T2 was cold rolled 85 it exhibited a greater tensile strength in the cold rolled state than T1 which was cold rolled only 50%. Generally steel T2 recovered at a faster rate than T1 so that at the onset of recrystallization, the steel had recovered about 30% or had a recovery fraction of about 0.7.

As can be noted from FIGURES l and 2, the titanium steels exhibit a constant increase in ductility during a recovery anneal independent of the time period of the anneal and independent of the annealing temperature.

It will be appreciated that similar graphical representations can be obtained by plotting yield stress as a function of time. It will also be understood that the composition of the titanium steel and the hot rolling practice will affect the cold rolled properties of the steel.

A plain low-carbon rimmed steel containing 0.07% carbon and no titanium was also prepared and processed in a manner generally similar to that described for the titanium-bearing steels, i.e., the steel after being made according to rimmed steel practice was hot rolled according to the usual practice for hot rolled strip or plate. Thereafter samples of the hot rolled product were reduced varying amounts between 50% and 85% by cold rolling and each of the cold rolled samples isothermally annealed at various temperatures between 850 F. and 1150 F. The solid lines in FIGURE 3 show the change in tensile strength and percent elongation with time for the rimmed steel samples cold rolled in amounts from 50% to 85 and isothermally recovery annealed at 85 F. The rimmed steel samples generally demonstrate an initial rapid decrease in tensile strength followed by a slight increase and then again a decrease. The percent elongation of the rimmed steel samples subjected to cold reductions greater than about 50% increases at first and then decreases corresponding generally with their increase in tensile strength. Thereafter the percent elongation again increases. It can be noted that for cold reductions between about 55% and 70%, the increase in percent elongation occurs quite rapidly after the initial decrease but for cold reductions greater than 70%, the increase in percent elongation occurs at a much lower rate after the initial decrease.

FIGURE 3 also includes, as dashed lines thereon, similar curves drawn for steels T1 and T2. In addition, FIG- URE 3 includes, also as a dashed line thereon, a curve illustrating the change in tensile strength with time during the annealing at 850 F. of a 70% reduced sample of the titanium-bearing steel described above. Only the curves showing the change in percent elongation for the and 85% cold reduced titanium steels (T1 and T2) are included on FIGURE 3, and it will be appreciated that the curves for intermediate cold reductions fall within the limits of these two curves. It will be noted that unlike the rimmed steels, the titanium steels exhibit an increase in ductility during recovery annealing independent of the time and temperature of annealing for cold reductions in excess of about 50% FIGURE 3 illustrates that for a cold reduction of about 50% both the tensile strength and percent elongation of the rimmed steel are higher at any time during a recovery anneal than are the tensile strength and percent elongation of the titanium steel. However, as the degree of cold reduction increases above 50% the percent elongation exhibited by the rimmed steel after a recovery anneal drops olf quite rapidly until at cold reductions of about 70% or higher, the percent elongation of the titanium steel remains above that of the rimmed steel at any point of time during the recovery anneal. Even for cold reductions of about the percent elongation of the titanium steel is higher for a substantial period of time during the heat treatment than that of the rimmed steel.

The cold reduced rimmed steel reaches an initial maximum percent elongation of about 4% after six seconds of annealing time and thereafter rapidly decreases in ductility until a minimum of about 1.5 percent elongation is reached. Thereafter the percent elongation slowly increases as the heat treatment continues until finally after about 400 minutes of annealing time the initial maximum percent elongation of 4% is again reached. By this time, however, the tensile strength of the steel has decreased to about 95,000 p.s.i. The titanium steel when cold reduced 70% on the other hand demonstrates a constant increase in ductility during recovery annealing so that at any time prior to the onset of recrystallization the titanium steel exhibits a higher value of percent elongation than does the rimmed steel. Further, the tensile strength of the titanium steel generally remains higher during the recovery anneal than does the tensile strength of the rimmed steel and even where the tensile strengths are about the same, the percent elongation of the rimmed steel is at a minimum as can be seen from FIGURE 3. These differences in the properties between the rimmed and titanium steels are accentuated for more severe cold reductions. Thus considering the cold reduced material it can be seen that the rimmed steel does not regain its initial maximum ductility even after annealing at 850 F. for 1,000 minutes while the titanium steel constantly increases in ductility during the anneal. Further, the 85% cold reduced titanium steel has a higher tensile strength at any time except where the rimmed steel demonstrates its minimum ductility. It can be seen, therefore, that my invention provides steel products having high tensile strengths and ductilities, the precise properties obtained for any given steel subjected to any given cold reduction being controlled by thelength of the recovery anneal at a particular temperature.

The relationships between the titanium and rimmed steels, as described above in reference to FIGURE 3, are maintained at annealing temperatures other than 850 F. In addition, generally the same relationships are evidenced when the titanium steels are compared to killed steels containing no titanium or titanium less than that required to provide a titanium to carbon ratio greater than 4 to l.

I claim:

1. A method of treating plain low-carbon titaniumbearing steel having an effective titanium to carbon ratio of at least 4 to 1 comprising cold reducing the steel, and thereafter heating the steel at a temperature and for a time period suflicient to increase its ductility, as measured by percent elongation without causing recrystallization to occur, said cold reducing being of a degree such that said increased ductility results independent of the temperature and time of heating.

5 6 2. The method of claim 1 wherein the steel is cold References Cited reduced by UNITED STATES PATENTS 3. The method of claim 1 wherein the heating step comprises a continuous anneal. f f 1 Wherem the heatmg Step 5 L. DEWAYNE RUTLEDGE, Primary Examiner 5. The method of claim 1 wherein the steel is cold re- W, W, STALLARD, A i t t E i duced at least about 70% by cold rolling.

6. The method of claim 1 wherein the steel is cold US. Cl. X.R. reduced at least about 50% by cold rolling. 10 148134 3,264,144 8/1966 Frazier et al. 148l2 

